If you can't tell your best friend your true feelings... they ain't your best friend. The vibe I get from reading this is that you two are very close and you want to date someone that close and not just be with randos.
the part that urks me as a man is if she's getting upset that you did something wrong for thinking she is high enough quality to date. As if you have betrayed her or something. Clearly you weren't just hanging out with her all this time for sex. If that were the case after 2 years you woulda moved on.
If I were you, id just set her down and tell her you aren't going to have any hidden agenda. tell her how you feel then ask her something along the lines of "what is it you are looking for? Do you want someone who treats you like I do? cause if so.. im right here."
But if you really aren't interested in her romantically and Ive read everything you said incorrectly, just apologize and suggest you hang out soon and move on with your friendship.